Taxpol and Advice Specialists advertise their services on the internet and also operate from offices across London.

Taxpol, which has offices in Greenford in west London and in Stratford in east London, lists a host of charges for a variety of claims for its clients, including £40 for child tax credit, £60 for child benefit, £90 for housing and council tax benefit, and £50 for the Sure Start Maternity Grant.

The fees include the cost of “filling in and posting” applications.

Advice Specialists, based in north-west London, offers a range of services for applications for benefits including a charge of between £30 and £140 for child benefit, £20 for child tax credit and £90 for housing and council tax benefit.

Advice Specialists’ website, which is written only in Polish, declares: “Our six years’ of experience in organising child benefits, housing allowances, allowance for people on low incomes and the recovery of overpaid taxes, speaks for itself.

"Thanks to our great experience we can look after you from the moment you cross the British border.”

Opponents of open European borders have complained that the UK’s benefits system is far too generous and too easily available for European Union migrants.

The European Commission denies that is the case and published a report earlier this month which suggested “benefit tourism” — the practice of coming to the UK to claim benefits — was “neither widespread nor systematic”.

Migration Watch UK, a think tank, published a report earlier this year that showed more than £1 million a week — about £55 million a year — is being paid out in child benefit for children who do not actually live in the UK.

Migration Watch said the UK was one of only five EU countries that did not require children to be resident in the host country in order to receive the payments. Of 40,171 children living outside the UK whose parents receive child benefit, 25,659 live in Poland (63 per cent).

According to Migration Watch, a family with two children is entitled to about £135 a month in benefits — the same as the minimum wage in Romania and Bulgaria, two of the poorest countries in the EU.

Restrictions on Romanians and Bulgarians coming to the UK to work will be removed on January 1.

Sir Andrew Green, Migration Watch’s chairman, said: “These payments of child benefits to children who may never have even set foot in Britain is yet another way in which the British benefits system encourages migration from the much poorer countries in eastern Europe.”

The two Polish-run firms, which are registered in the UK, also offer other accounting services such as helping self-employed workers fill out tax returns. Taxpol says on its website that helping with “self-employment” tax returns “constitutes the largest part of our company services”.

Artur Saniuk, the owner of Advice Specialists, said he had several hundred clients and “for 10 per cent we do these benefits”.

Mr Saniuk said: “The child benefit is a lot of money but living here is so expensive. Half the wages go on cost of living and there is not much left to send to Poland. An English person living in the UK has the right to child benefit so why shouldn’t the eastern European person get that benefit.”

Mr Saniuk insisted he did not always charge for arranging benefits but offered it free with his accounting service.

A spokesman for HM Revenue and Customs said: “Under EC social security rules, EEA (European Economic Area) migrant workers can claim child tax credit and child benefit for children they are responsible for residing in another member state.”